The present invention relates to a device for suppressing generation of radio wave noise from ignition systems and, more particularly, to a radio-wave-noise suppression distributor used in an ignition system for internal combustion engine, in which distributor radio wave noise caused from the rotor electrode and the counter electrodes of the distributor is minimized.
Hitherto, an ignition distributor has been proposed in which a high-resistance layer is formed by plasma spraying process on the end of the rotor electrode for the purpose of reducing the radio wave noises. This type of the distributor rotor is generally referred to as "plasma-spray-coated rotor".
More specifically, in this plasma-spray-coated rotor, a layer of high-resistance electrode is formed on at least one of the discharge electrode on the rotor and the discharge electrodes of the counter electrodes as disclosed in U.S. Pat. No. 4,007,342. The high-resistance layer is formed, as proposed in U.S. Pat. No. 3,992,230, by spraying a metal oxide having a high electric resistance, e.g., CuO, onto the discharge electrode by means of plasma.
The ignition distributor having the sprayed electrode provided with the CuO layer, however, suffers from a disadvantage in that CuO constituting the high-resistance layer is reduced to Cu.sub.2 O when used in the atmosphere of high temperature, resulting in a reduced electric resistance of the high-resistance layer, so that the noise reducing effect of the layer is lost. In order to overcome this problem, it has been proposed to add a metal oxide which is stable even in the atmosphere of high temperature, e.g., Al.sub.2 O.sub.3, SiO.sub.2, to the material of the high-resistance layer. Such a countermeasure is disclosed, for example, in U.S. Pat. No. 4,175,144.
A distributor has been proposed also in which a layer of a composite material of a mixture of a metal such as Cu and a metal oxide such as Al.sub.2 O.sub.3 is formed on the end of the electrode, as shown, for example, in Japanese Laid-Open Patent Publication No. 87859/1980. This type of distributor, however, does not have sufficient effect of suppression of radio wave noise, although it can lower the discharge starting voltage. In addition, this type of distributor is impractical in that it is difficult to produce. In addition, the effect of lowering the discharge starting voltage could not be obtained stably because the layer formed on the electrode cannot have stable grain boundary structure, unlike the plasma-spray-coated rotor.
On the other hand, there is a trend for smaller height of the bonnet of motor vehicles due to an increasing demand for front-engine front-wheel driving type vehicles, as well as for higher aerodynamic performance. This in turn requires the clearance between the distributor and the bonnet to be reduced, causing a tendency of higher level of radio wave noises from the distributor. Under this circumstance, there is an increasing demand for higher noise prevention effect of plasma-spray-coated rotors.